There once existed a 'quench severity' list. It rated iced brine as 125% as severe as cold, clean water.
Anywayz... I like the home chemistry. Inventors in there own kitchens and sheds will always hold a special place in this engineer's heart, for they embody the true spirit of engineering. The idea's not new, but this seems the most refined I've seen so far.
I'll elaborate on my original example for clarification:
"What influences the hardness of a steel when quenched is the carbon content with respect to the eutectoid point of that particular composition. In a plain carbon steel, this is just under 0.8%C. In 41xx alloys, that's about 0.6% carbon (the alloy solutes, just like an other solution system lower each other's systematic solubility, so the eutectoid carbon content is usually lowered in alloy steels)."
That statement is usually simplified to; "Carbon is the only element that affects the hardness of martensite". And it's the most important part of ferrous metallurgy to remember.
No matter what you do, no matter how hard you quench, there's an absolute hardness you can reach with any given carbon content. At 0.3%C (in a 4130), it's about 55Rc. Most cases, even in thin section, you'll never get above 50. Now, how deep that hardness goes, before it starts to taper off from thermal effects is hardenability - how easy it is to harden and that is controlled solely be everything but carbon.
In short:
Carbon controls how hard
Alloying controls how deep
God, this sounds filthy ;-)
So, even in a superquench® you're limited to a certain hardness hard and to a certain depth. But it's still air hardening, and it does air harden. Just because it doesn't reach 60Rc....